SI-6103 & SI-8592 signatures of mixed in methods should use instantiated type params

[adriaanm]

• SI-6103 Mixin generates method signatures that confuse javac
• SI-8592 mixin method overrides final method with variation on the reflect.{api,internal} pattern of abstract types implemented by a class

[adriaanm]

This has become less pressing now that we don't mix in forwarders in subclasses, which implies we're not mixing in forwarders with the wrong signature :-) Fixing this properly would require doing (some of) mixin before (some of) erasure (we need more bridges after mixing in methods with sharper (erased) signatures than the (erased) one in the supertrait).

[smarter]

This has become less pressing now that we don't mix in forwarders in subclasses

Not true anymore since -Xmixin-force-forwarders was set to true by default for performance reasons, right ?